A '68 will interchange about 95% of parts with your '69. There are some minor differences, like round vs. rectangular side-marker lights, etc.
A 1970 - 72 will fit most of the undercarriage parts (drivetrain, rearend, shocks, control arms, etc.) but the body & interior are pretty different. This is pretty much also true for 1968 - '72 Olds & Pontiacs too, depending on the particular item.